1. Field of the Invention
A characteristic of seed development in most plants is the accumulation of storage compounds such as proteins, sugars and oil. This invention relates generally to nucleic acid sequences encoding proteins that are related to the presence of seed storage compounds in plants. More specifically, the present invention relates to nucleic acid sequences encoding sugar and lipid metabolism regulator proteins and the use of these sequences in transgenic plants.
2. Background Art
The study and genetic manipulation of plants has a long history that began even before the famed studies of Gregor Mendel. In perfecting this science, scientists have accomplished modification of particular traits in plants ranging from potato tubers having increased starch content to oilseed plants such as canola and sunflower having increased or altered fatty acid content. With the increased consumption and use of plant oils, the modification of seed oil content and seed oil levels has become increasingly widespread (e.g. Töpfer et al. 1995, Science 268: 681-686). Manipulation of biosynthetic pathways in transgenic plants provides a number of opportunities for molecular biologists and plant biochemists to affect plant metabolism giving rise to the production of specific higher-value products. The seed oil production or composition has been altered in numerous traditional oilseed plants such as soybean (U.S. Pat. No. 5,955,650), canola (U.S. Pat. No. 5,955,650), sunflower (U.S. Pat. No. 6,084,164) and rapeseed (Töpfer et al. 1995, Science 268: 681-686), and non-traditional oil seed plants such as tobacco (Cahoon et al. 1992, Proc. Natl. Acad. Sci. USA 89: 11184-11188).
Plant seed oils comprise both neutral and polar lipids (see Table 1). The neutral lipids contain primarily triacylglycerol, which is the main storage lipid that accumulates in oil bodies in seeds. The polar lipids are mainly found in the various membranes of the seed cells, e.g. the endoplasmic reticulum, microsomal membranes and the cell membrane. The neutral and polar lipids contain several common fatty acids (see Table 2) and a range of less common fatty acids. The fatty acid composition of membrane lipids is highly regulated and only a select number of fatty acids are found in membrane lipids. On the other hand, a large number of unusual fatty acids can be incorporated into the neutral storage lipids in seeds of many plant species (Van de Loo F. J. et al. 1993, Unusual Fatty Acids in Lipid Metabolism in Plants pp. 91-126, editor T S Moore Jr. CRC Press; Millar et al. 2000, Trends Plant Sci. 5: 95-101).
TABLE 1Plant Lipid ClassesNeutral LipidsTriacylglycerol (TAG)Diacylglycerol (DAG)Monoacylglycerol (MAG)Polar LipidsMonogalactosyldiacylglycerol (MGDG)Digalactosyldiacylglycerol (DGDG)Phosphatidylglycerol (PG)Phosphatidylcholine (PC)Phosphatidylethanolamine (PE)Phosphatidylinositol (PI)Phosphatidylserine (PS)Sulfoquinovosyldiacylglycerol
TABLE 2Common Plant Fatty Acids16:0Palmitic acid16:1Palmitoleic acid16:3Palmitolenic acid18:0Stearic acid18:1Oleic acid18:2Linoleic acid18:3Linolenic acidγ-18:3   Gamma-linolenic acid*20:0Arachidic acid22:6Docosahexanoic acid (DHA)*20:2Eicosadienoic acid20:4Arachidonic acid (AA)*20:5Eicosapentaenoic acid (EPA)*22:1Erucic acid*These fat acids do not normally occur in plant seed oils, but their production in transgenic plant seed oil is of importance in plant biotechnology.
Lipids are synthesized from fatty acids and their synthesis may be divided into two parts: the prokaryotic and the eukaryotic pathway (Browse et al. 1986, Biochemical J. 235: 25-31; Ohlrogge & Browse 1995, Plant Cell 7: 957-970). The prokaryotic pathway is located in plastids that are the primary site of fatty acid biosynthesis. Fatty acid synthesis begins with the conversion of acetyl-CoA to malonyl-CoA by acetyl-CoA carboxylase (ACCase). Malonyl-CoA is converted to malonyl-acyl carrier protein (ACP) by the malonyl-CoA:ACP transacylase. The enzyme beta-keto-acyl-ACP-synthase III (KAS III) catalyzes a condensation reaction in which the acyl group from acetyl-CoA is transferred to malonyl-ACP to form 3-ketobutyryl-ACP. In a subsequent series of condensation, reduction and dehydration reactions, the nascent fatty acid chain on the ACP cofactor is elongated by the step-by-step addition (condensation) of two carbon atoms donated by malonyl-ACP until a 16- or 18-carbon saturated fatty acid chain is formed. The plastidial delta-9 acyl-ACP desaturase introduces the first unsaturated double bond into the fatty acid. Thioesterases cleave the fatty acids from the ACP cofactor and free fatty acids are exported to the cytoplasm where they participate as fatty acyl-CoA esters in the eukaryotic pathway. In this pathway, the fatty acids are esterified by glycerol-3-phosphate acyltransferase and lysophosphatidic acid acyltransferase to the sn-1 and sn-2 positions of glycerol-3-phosphate, respectively, to yield phosphatidic acid (PA). The PA is the precursor for other polar and neutral lipids, the latter being formed in the Kennedy pathway (Voelker 1996, Genetic Engineering ed.: Setlow 18: 111-113; Shanklin & Cahoon 1998, Annu. Rev. Plant Physiol. Plant Mol. Biol. 49: 611-641; Frentzen 1998, Lipids 100: 161-166; Millar et al. 2000, Trends Plant Sci. 5: 95-101).
Acetyl-CoA in the plastids is the central precursor for lipid biosynthesis. Acetyl-CoA can be formed in the plastids by different reactions and the exact contribution of each reaction is still being debated (Ohlrogge & Browse 1995, Plant Cell 7: 957-970). It is however accepted that a large part of the acetyl-CoA is derived from glucose-6-phosphate and pyruvate that are imported from the cytoplasm into the plastids. Sucrose is produced in the source organs (leaves, or anywhere that photosynthesis occurs) and is transported to the developing seeds that are also termed sink organs. In the developing seeds, the sucrose is the precursor for all the storage compounds, i.e. starch, lipids and partly the seed storage proteins. Therefore, it is clear that carbohydrate metabolism in which sucrose plays a central role is very important to the accumulation of seed storage compounds.
Although lipid and fatty acid content of seed oil can be modified by the traditional methods of plant breeding, the advent of recombinant DNA technology has allowed for easier manipulation of the seed oil content of a plant, and in some cases, has allowed for the alteration of seed oils in ways that could not be accomplished by breeding alone (see, e.g., Töpfer et al. 1995, Science 268: 681-686). For example, introduction of a Δ12-hydroxylase nucleic acid sequence into transgenic tobacco resulted in the introduction of a novel fatty acid, ricinoleic acid, into the tobacco seed oil (Van de Loo et al. 1995, Proc. Natl. Acad. Sci. USA 92: 6743-6747). Tobacco plants have also been engineered to produce low levels of petroselinic acid by the introduction and expression of an acyl-ACP desaturase from coriander (Cahoon et al. 1992, Proc. Natl. Acad. Sci USA 89: 11184-11188).
The modification of seed oil content in plants has significant medical, nutritional and economic ramifications. With regard to the medical ramifications, the long chain fatty acids (C18 and longer) found in many seed oils have been linked to reductions in hypercholesterolemia and other clinical disorders related to coronary heart disease (Brenner 1976, Adv. Exp. Med. Biol. 83: 85-101). Therefore, consumption of a plant having increased levels of these types of fatty acids may reduce the risk of heart disease. Enhanced levels of seed oil content also increase large-scale production and thereby reduce the cost of these oils.
In order to increase or alter the levels of compounds such as seed oils in plants, nucleic acid sequences and proteins regulating lipid and fatty acid metabolism must be identified. As mentioned earlier, several desaturase nucleic acids such as the Δ6-desaturase nucleic acid, Δ12-desaturase nucleic acid and acyl-ACP desaturase nucleic acid have been cloned and demonstrated to encode enzymes required for fatty acid synthesis in various plant species. Oleosin nucleic acid sequences from such different species as Brassica, soybean, carrot, pine and Arabidopsis thaliana have also been cloned and determined to encode proteins associated with the phospholipid monolayer membrane of oil bodies in those plants.
Storage lipids in seeds are synthesized from carbohydrate derived precursors. Plants do have a complete glycolytic pathway in the cytosol (Plaxton 1996, Annu. Rev. Plant Physiol. Plant Mol. Biol. 47: 185-214) and it has been shown that a complete pathway also exists in the plastids of rapeseeds (Kang & Rawsthorne 1994, Plant J. 6: 795-805). Sucrose is the primary source of carbon and energy, transported from the leaves into the developing seeds. During the storage phase of seeds, sucrose is converted in the cytosol to provide the metabolic precursors glucose-6-phosphate and pyruvate. These are transported into the plastids and converted into acetyl-CoA that serves as the primary precursor for the synthesis of fatty acids. Although several nucleic acids that are involved in enzymatic steps of the metabolism of lipids, fatty acids and starch have been cloned and identified, there are likely a multitude of such plant nucleic acids that have yet to be identified. Phenotypic analysis of several oilseed plants and other mutated plants has revealed other putative proteins involved in plant lipid metabolism, but the prior art has yet to describe the genomic location of these proteins or the sequence of the nucleic acids that encode them.
An exemplary study is that of the oilseed plant Arabidopsis thaliana. Focks and Benning (1998, Plant Physiol. 118: 91-101) isolated and characterized a wrinkled mutant of Arabidopsis thaliana designated wri1. The wri1 mutant has a decreased seed oil content that was speculated to be due to a defect in the seed-specific regulation of carbohydrate metabolism. In the wri1 mutant, the activities of several glycoltic enzymes were reduced and the mutant seeds were impaired in the incorporation of sucrose and glucose into triacylglycerol lipids, while important precursor molecules for plastidial lipid biosynthesis, like pyruvate and acetate, were incorporated at increased rates. This biochemical evidence was interpreted by Focks & Benning (1998, Plant Physiol. 118: 91-101) as indication that the WRI1 protein could be a regulatory protein governing carbohydrate metabolism during seed development.
The regulation of protein phosphorylation by kinases and phosphatases is accepted as a universal mechanism of cellular control (Cohen 1992, Trends Biochem. Sci, 17: 408-413), and Ca2+ and calmodulin signals are frequently transduced via Ca2+ and calmodulin-dependent kinases and phosphatases (Roberts & Harmon 1992, Annu. Rev. Plant Physiol. Plant Mol. Biol. 43: 375-414.). Okadaic acid, a protein phosphatase inhibitor, has been found to affect both gibberellic (GA) and absisic acid (ABA) pathways (Kuo et al. 1996, Plant Cell. 8: 259-269). Although the molecular basis of GA and ABA signal transduction remains poorly understood, it seems well established that the two phytohormones are involved in overall regulatory processes in seed development (e.g. Ritchie & Gilroy 1998, Plant Physiol. 116: 765-776).
There is a clear need to specifically identify factors that are more specific for the developmental regulation of storage compound accumulation. In order to find specific key regulatory genes controlling seed oil and sugar biosynthesis, transcription factors, protein kinases and phosphates provide proteins which can alter seed storage compound production. Elucidating the function of genes directly and/or indirectly involved in oil production provides important information for designing new strategies for crop improvement. There is a need, therefore, to identify genes which have the capacity to confer altered or increased oil production to its host plant and to other plant species. Particularly well suited plants for this purpose are oilseed plants containing high amounts of lipid compounds like rapeseed, canola, linseed, soybean, sunflower maize, oat, rye, barley, wheat, sugarbeet, tagetes, cotton, oil palm, coconut palm, flax, castor and peanut, for example.